My wife and I have a disagreement about a practice of mine--engine braking. When I'm driving her manual transmission, I make pretty extensive use of the technique, occasionally when approaching an stoplight, but especially (and relevant to this discussion), when going down long steep hills. Tonight I was going down one and even bringing it down to 3rd gear I was still accelerating, so I went down to 2nd, and the car held steady around 45 mph. However, the tach also jumped up, and my wife found the noise disconcerting. My defense was that we were still a good 1500 rpm short of the red line, and the temperature gauge was flat, so there's nothing to be concerned about. She was not convinced.

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We've had similar questions on topics like this that have since been closed as off-topic (focused on driving rather than maintenance): mechanics.stackexchange.com/a/681/57 Your car can over-rev on a steep hill, though.
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Bob Cross♦May 28 '12 at 4:06

I should say, I'm asking this from a maintenance perspective; e.g., what effect would it have on the engine, transmission, etc
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RayMay 28 '12 at 11:39

4 Answers
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Answer in progress. (Shall expand, and add more authoritative links when have time later).

Fuel Consumption

Modern electronic (as opposed to mechanical) fuel-injection systems (that also includes TBI (throttle body) single-point injection) are equipped with throttle position sensor. In the event of overrun (higher RPM, closed throttle) fuel input is cut off, thus making it more efficient than coasting in neutral and using brakes alone (one supposed to brake with gears engaged anyway). Fuel kicks in only when engine speed (RPM) approaches to or below the idle speed to maintain it.

Fuel metering is interrupted during trailing throttle [overrun]. Although this expedient saves fuel on downhill stretches, its primary purpose is to guard the catalytic converter against overheating stemming from poor and incomplete combustion (misfiring)

[…]

Cutoff of the fuel supply during overrun operation permits the fuel consumption to be reduced considerably not only when driving downhill but also in town traffic.

Similar data can be found on systems of other manufacturers. Some of them even allow the cut-off parameters to be modified (see adjustment of overrun for SManager software for s300 module for Honda ECUs — good illustration on how this feature works).

Engine Wear

As above suggests, power stroke is eliminated, ergo one of the most demanding energy loads on the engine is gone. In all, given proper care and maintenance, consensus is that engine braking does not add any statistically significant friction wear on the motor itself.

To test this hypothesis I did several searches on the subject via academic databases and Google Scholar (both with and without patents), and I have not found a single paper concerned with increased engine wear, but plenty discussing the methods to increase effectiveness of engine braking, as power of modern engines increased dramatically, and drive-train losses are reduced. As this U.S. Patent 5,146,890 (by Volvo) states (p.1 of “Description”):

When driving in hilly terrain, the wheel brakes should be used as little as possible, primarily for safety reasons. The average speed of the vehicle in hilly terrain is therefore greatly influenced by the available engine braking power, which increases the requirement for a more effective engine brake that will also be capable of reducing wear and tear on the wheel brakes and thereby improve running economy.

Gearbox Wear

Higher RPM by themselves do not mean that gearbox is being pushed beyond its design limitations. Few hills at higher RPM due to engine braking (given smooth transitions when switching) would not cause any more wear than, say, hours on end on the motorway pushing over 120 km/h (75 mph). If mountain roads is your primary area of operation then it would qualify as severe use (just like frequent towing), and would require transmission cooler anyway.

"virtually shut off" is still greater than zero. RPMS will determine the number of injections from the fuel system: higher revs will always net a higher fuel consumption even in the closed throttle position. The exhaust stroke is the source of the greatest risk of fatigue failure. For an explanation why, see Maximum Boost (Corky Bell), Figure 1.5.
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Bob Cross♦May 29 '12 at 21:47

@Bob As you can see in technical literature, it is not “virtually”, it is “completely”. I was just using cautious language (academic habit) to hedge my bets, before I knew better (now I do, as this feature seems to be universal).
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theUgMay 29 '12 at 22:35

@Bob I am pretty sure that automotive engineers designing these FI systems did so within the limitations and tolerances of the engines (including possible eventualities like losing fuel or spark at 6000 RPM) so much so they are more concerned about the damage done to the catalytic converter due to incomplete combustion, rather than conrod fatigue failure in the absence of the negating effects of the gas expansion force. Given the above, I still do not see that down-vote was warranted.
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theUgMay 29 '12 at 22:36

Your discussion of fuel injection is irrelevant to the original question regarding the risks induced by the mechanically-driven acceleration of the moving parts of the engine due to the steep grade.
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Bob Cross♦May 30 '12 at 0:11

@Bob: On the contrary: the book you had used as an example (I had read the chapter you had referenced) explains how compressive force of the expanding gas negates to some extent tensile force of inertia. you seem to imply that in the absence of that negating force, fatigue failure becomes a clear risk. I argue that these kinds of risks are well within engineering tolerances of the engine, so they should not be an issue. And as everything is obviously over-engineered, one should not expect pistons flying out through the bonnet the instance one hits a red line.
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theUgMay 30 '12 at 5:05

It's not going to overheat the car. No fuel will be injected assuming the car meets the fuel cut criteria (warmed up, RPM > 1500 or so, depends on the car). It will cool it down since relatively cool air is not being heated by combustion (albeit compression is still heating it a bit). This cools the cylinders and exhaust which in turn cools the engine's coolant.

On a steep enough grade, the leverage of the wheels will continue to speed the engine to and past the redline. None of the ignition or fuel systems will be able to do anything about it. Something dramatically bad will happen to the engine at that point.

There will also be second-order effects like increased temperature in the gearbox and differentials but that is a natural consequence of running at higher revs. There will, of course, be significantly more noise. If your passengers are complaining (and it's their car?), that might be something to consider....

You have to make the call about how high is too high. If you were driving up and down Mount Washington, the guides insist that you keep the car in second gear. In that situation, you have to tolerate some higher revs and engine temperatures (it got surprisingly hot in the cabin when we went).

As a final factor in consideration: lower revs are more fuel efficient, engine braking or not.

When engine-braking, your throttle is off, and the vehicle should use less fuel than maintaining at the same RPM. Many (modern) fuel-injection systems would completely shut off fuel in this scenario, and let gravity do the job of maintaining engine speed. I do not want to say all vehicles/systems, but to be sure one needs to use trip computer or OBD performance monitor to see real time fuel consumption for particular model. Obviously, it is different for carburettor-based engines.
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theUgMay 28 '12 at 18:24

And, in case of acceleration, I have myself noticed after comparing several fuel-ups on my motorcycle, lugging the the motor (switching gears early and letting the engine work its way up), produced significantly worse results than using manufacturer-recommended higher switching RPM. In both cases I was taking it easy on the throttle, taking my time to achieve goal speed. Basically, what it means, direct proportion between RPM and fuel consumption only holds true for maintaining speed, not acceleration (positive or negative).
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theUgMay 28 '12 at 19:43

Another thing I had found during research. FI systems also have feature of shutting off fuel at red-line anyway, regardless of throttle opening.
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theUgMay 29 '12 at 20:26

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@theUg: Shutting off fuel at the redline won't help if it's not fuel that's spinning the engine.
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LnxPrgr3May 29 '12 at 21:20

@Lnx: It helps by not adding more accelerative force to the equation. Either way, you are both missing the point. We are not talking about runaway box of death flying down the road. We are talking about maintaining the speed on the downgrade, or, at least, reducing the acceleration to relieve wheel brakes from some of the work they have to do.
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theUgMay 30 '12 at 16:40

How high is "too high" for RPMs? My rule-of-thumb is to go down hills in the same gear I'd use to go up them.

As for wear, it should be pointed out that brakes are cheaper than clutches or engine re-builds. But as others have pointed out, the wear on an engine that is efficiently removing friction heat is likely to be much less than that on brakes that don't remove heat particularly efficiently.

But if you pump brakes (I use two seconds on, two seconds off), then the brakes can stay cool. If you can't get in the habit of pumping brakes, then you should be using engine braking.

So my answer is to go down in a high gear if the hill is shallow enough to allow you to control speed by brake-pumping, and go down in the gear you'd go up in if the hill is too steep for brake-pumping to control your speed.

When you pumping brakes, you are still using engine braking (as you should use your brakes with gears engaged). Unless, of course, you simultaneously put car in neutral, and/or open up throttle at the same time.
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theUgMay 29 '12 at 19:40

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Also, if your speed is the same in either case, pumping the brakes doesn't keep them cooler. In fact, it will raise them to a higher peak temp, while the average temp will be the same in both cases. Pumping also forces repeated thermal cycling, which is much worse than heat alone. In short: cut it out. :)
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Colin KMay 31 '12 at 17:30